German Patent No. 41 12 388 (U.S. Pat. No. 5,281,012) discloses a method and an apparatus for regulating or controlling braking force distribution in a motor vehicle on the basis of the wheel speeds of the front and rear wheels. If this electronic control system fails for any reason, undesirable driving situations can result in certain braking circumstances due to overbraking of the rear axle.
An emergency mode has therefore been proposed, for example in German Patent No. 1 95 11 152, for the case in which two speed sensors have failed simultaneously. In this, a saturation pressure value which is below the inlet pressure defined by the driver is established at the rear-wheel brakes by alternating pressure buildup and pressure reduction.
Further features for controlling or regulating the braking force distribution between the front and rear axles of a motor vehicle in the event of faults are described in German Patent No. 1 95 11 161 and German Patent No. 1 95 11 162.
In German Patent No. 1 95 11 161, the braking force distribution control system is deactivated in the event of faults within the control unit, and otherwise continues to operate, at least to a restricted extent, in the event of individual faults. For example, in the event of a defect in a speed sensor on a rear wheel or a front wheel, control is continued on the basis of a replacement signal. Control also continues in the event of defects in an actuator which lead to an undiminished brake pressure buildup at a front wheel, faults in the area of the return flow pump, faults in an actuator which lead to an inability to reduce braking force, or in the event of undervoltage. In the event of defects in an actuator which lead to an absence of pressure buildup at a front wheel, defects in an actuator which lead to an undiminished brake pressure buildup at a rear wheel, the controller is switched to passive mode.
German Patent No. 1 95 11 162 depicts emergency features of a braking force distribution control system in the event of faulty rotation speed signals. In this, in the event of a fault in a front-wheel rotation speed signal of the control system, the functional front-wheel rotation speed signal is taken as the basis, or the fastest of all the remaining wheels is utilized; in the event of a fault in a rear-wheel rotation speed sensor, the vehicle speed reference signal is used as the substitute signal. In addition, the sensitivity of the braking force distribution controller is increased in the event of a fault.
Methods for fault detection are described, for example, in SAE Paper 890870, "Electronic Control Unit of the Sumitomo Electronic Antilock System."
The conventional procedures for maintaining the braking force distribution regulation or control system in the event of a fault manage the individual faults described therein reliably and effectively. Detailed emergency features of the braking force distribution controller are not described, in particular in the event of a fault in the brake pedal switch whose position is analyzed as an indicator of a braking action and thus for activation of the controller.